/*
 * Copyright 2019 Google LLC
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.android.apps.authenticator.otp;

import android.content.Context;
import android.content.SharedPreferences;
import android.preference.PreferenceManager;
import com.google.android.apps.authenticator.time.Clock;
import com.google.android.apps.authenticator.util.Utilities;
import com.google.common.annotations.VisibleForTesting;

/**
 * Clock input for the time-based OTPs (TOTP).
 *
 * <p>The input is based on the current system time and is adjusted by a persistently stored
 * correction value (offset in minutes).
 */
public class TotpClock implements Clock {
  
  @VisibleForTesting
  static final String PREFERENCE_KEY_OFFSET_MINUTES = "timeCorrectionMinutes";
  
  private final Clock mSystemWallClock;
  private final SharedPreferences mPreferences;
  
  private final Object mLock = new Object();
  
  /**
   * Cached value of time correction (in minutes) or {@code null} if not cached. The value is cached
   * because it's read very frequently (once every 100ms) and is modified very infrequently.
   * 
   * @GuardedBy {@link #mLock}
   */
  private Integer mCachedCorrectionMinutes;
  
  public TotpClock(Context context, Clock systemWallClock) {
    mSystemWallClock = systemWallClock;
    mPreferences = PreferenceManager.getDefaultSharedPreferences(context);
  }
  
  @Override
  public long nowMillis() {
    return mSystemWallClock.nowMillis() + getTimeCorrectionMinutes() * Utilities.MINUTE_IN_MILLIS;
  }
  
  /**
   * Gets the currently used time correction value.
   * 
   * @return number of minutes by which this device is behind the correct time.
   */
  public int getTimeCorrectionMinutes() {
    synchronized (mLock) {
      if (mCachedCorrectionMinutes == null) {
        mCachedCorrectionMinutes = mPreferences.getInt(PREFERENCE_KEY_OFFSET_MINUTES, 0); 
      }
      return mCachedCorrectionMinutes;
    }
  }
  
  /**
   * Sets the currently used time correction value.
   * 
   * @param minutes number of minutes by which this device is behind the correct time.
   */
  public void setTimeCorrectionMinutes(int minutes) {
    synchronized (mLock) {
      mPreferences.edit().putInt(PREFERENCE_KEY_OFFSET_MINUTES, minutes).commit();
      // Invalidate the cache to force reading actual settings from time to time
      mCachedCorrectionMinutes = null;
    }
  }
  
  /**
   * Gets the system "wall" clock on top of this this TOTP clock operates.
   */
  public Clock getSystemWallClock() {
    return mSystemWallClock;
  }
}
